An ion source is a critical component of an ion implanter and other processing equipment. The ion source generally includes an arc chamber which accepts a feed gas. The feed gas is ionized in the arc chamber by differing techniques known in the art to generate plasma. The plasma is generally a quasi-neutral collection of ions (usually having a positive charge) and electrons (having a negative charge). The plasma has an electric field of about 0 volts per centimeter in the bulk of the plasma. The plasma is bounded by a region generally referred to as a plasma sheath. The plasma sheath is a region that has fewer electrons than the plasma. The light emission from this plasma sheath is less intense than the plasma since fewer electrons are present and hence few excitation-relaxation collisions occur. Hence, the plasma sheath is sometimes referred to as “dark space.”
Turning to FIG. 1, a cross sectional view of a known ion source 100 is illustrated having an arc chamber 102. The arc chamber 102 includes a sidewall 103 having an extraction aperture 110. A feed gas (not illustrated) is ionized in the arc chamber 102 to generate plasma 140 having a plasma sheath 142 proximate the extraction aperture 110. The boundary 141 between the plasma 140 and the plasma sheath 142 proximate the extraction aperture is generally parallel to a plane 132 defined by an interior surface of the sidewall 103 depending on the plasma density of the plasma 140 and the electric field generated by an extraction electrode assembly (not illustrated). Ions 106 are extracted by the extraction electrode assembly into a well-defined ion beam 118.
One drawback with this conventional ion source is the lack of ion beam focusing originating from the extraction aperture 110. Ions 106 are accelerated across the plasma sheath 142 at about right angles to the boundary 141 between the plasma 140 and the plasma sheath 142. Since the boundary 141 is generally parallel to the plane 132, there is little angular spread control of the ions 106 that form the ion beam 118. Another drawback with this conventional ion source is the shape of the boundary 141 limits the number of ions 106 that can be accelerated across the same through the extraction aperture 110. This can limit the ion beam current density achievable from the ion source. Ion beam current density is a beam current value per unit area typically expressed in milliamperes per square centimeters (mA/cm2). A relatively higher beam current density is desirable in some instances and can improve the throughput performance of a given process. Yet another drawback with a conventional ion source is that the shape of the boundary 141 is determined by the plasma density of the plasma 140 and the strength of the electric field. For example, for a given plasma density, a high extraction electric field may cause a concave boundary. A decrease in plasma density may cause a convex boundary. All these facts limit emittance control of the ion beam from the conventional ion source. The emittance of an ion beam is generally spatial and angular distributions of the ion beam, and can also be defined roughly as the product of beam diameter and angular spread in transverse momentum at each point.
Accordingly, there is a need for an ion source which overcomes the above-described inadequacies and shortcomings.